The invention relates to pressure swing adsorption (PSA) processes and apparatus, more particularly to the use of high performance adsorbents in PSA processes and systems through the novel deployment of such adsorbents in layers.
Cryogenic methods have dominated air separation processes for many years where high purity O2, N2 and or Ar are desired. More recently, both membrane and adsorption processes have become important commercially. In particular, PSA, including superatmospheric adsorption/desorption processes, subatmospheric vacuum swing adsorption (VSA) and transatmospheric vacuum pressure swing adsorption (VPSA) processes are well known in the art. Such methods are typically used to produce oxygen having a purity between about 90 to 95%. There is an increasing need for this purity O2 in such diverse industries as steel making, glass making and pulp and paper production. Single plant oxygen capacity for such adsorption processes now exceeds 100 tons-per-day contained O2 (TPDO), and applications continue to arise demanding even greater capacities. At these production and purity levels, O2 product cost is lower by adsorption than by cryogenic methods, while for larger capacities, economies of scale currently favor the cryogenic methods. Nevertheless, there continues to be considerable economic incentive to extend the production range of adsorption processes for air separation. This must be accomplished by improving performance while reducing the cost of power and capital.
A typical adsorption system for the production of O2 includes one or more adsorber vessels containing a layer of pretreatment adsorbent for removing atmospheric contaminants followed by a main adsorbent. The pretreatment adsorbent can be any material primarily effective in removing H2O and CO2, e.g. zeolites, activated alumina, silica gel, activated carbon and other such adsorbents. The main adsorbent material, which usually represents at least 90% of the total volume of adsorbent in the vessel is N2xe2x80x94selective, typically from the type A or type X family of zeolites. While many different adsorption cycles have been developed for O2 production, all pressure swing cycles contain the four basic steps of pressurization, adsorption, depressurization and desorption. When multiple beds are used, the beds are sequenced out of phase for the different cycle steps in order to maintain a constant flow of product. One of many examples of such processes illustrating these basic features is given by Batta in U.S. Pat. No. 3,636,679.
There has been significant development of the various PSA, VSA and VPSA methods for air separation over the past thirty years, with major advances occurring during the last decade. Commercialization of these processes and continued extension of the production range can be attributed primarily to improvements in the adsorbents and process cycles, with advances in adsorber design contributing to a lesser degree. Highly exchanged lithium molecular sieve adsorbents, as illustrated by Chao in U.S. Pat. No. 4,859,217, are representative of advanced adsorbents for O2 production. A historical review of both adsorbent and process cycle development may be found in Kumar (Sep. Sci. and Technology, 1996).
The increase in N2/O2 selectivity and N2 working capacity associated with N2-selective advanced adsorbents is largely responsible for the improvements in O2 recovery and reduction in power and bed size factor (BSF). Such adsorbents, however, often have higher heats of adsorption, are more difficult to manufacture and may have poorer mass transfer characteristics, all resulting in a higher adsorbent cost. While many new adsorbents have been developed claiming improved properties for air separation, only a few have been implemented successfully in commercial processes. Advanced adsorbents often fail or fall short of expectations since process performance is projected on the basis of adsorbent equilibrium properties and isothermal-process conditions.
Collins in U.S. Pat. No. 4,026,680 teaches that adiabatic operation intensifies the thermal effects in the adsorbent bed inlet zone. In particular he teaches that there is a xe2x80x9csharply depressed temperature zone,xe2x80x9d (hereinafter referred to as a xe2x80x9ccold zonexe2x80x9d), in the adsorption bed inlet end. This zone is as much as 100xc2x0 F. below the feed gas temperature. Such a zone results in a thermal gradient over the length of the adsorbent bed of approximately the same magnitude (e.g. about 100xc2x0 F.). Collins suggests that the cold zone arises from the coupling of an xe2x80x9cinadvertent heat-regenerative stepxe2x80x9d at the inlet end of the bed with the thermal cycling resulting from the adsorption/desorption steps of the process. The regenerative effect may be partly the result of the adsorption of water vapor and carbon dioxide in a pretreatment zone located ahead of the main adsorbent.
The thermal cycling that occurs in an adiabatic process results in an adverse thermal swing, i.e. the adsorption step occurs at a higher temperature than the desorption step. This thermal swing tends to increase with increasing adsorbate/adsorbent heats of adsorption and increasing ratio of adsorption to desorption pressure. These gradients and swings in bed temperature result in various parts of the adsorbent bed functioning at different temperatures. The N2/O2 selectivity and N2 working capacity of any particular adsorbent may not be effectively utilized over such wide ranges in bed temperature. Dynamic adsorbent properties that vary strongly with temperature are also likely to result in process instability when operating conditions, such as ambient temperature, change.
Considerable attention has been given to eliminating or minimizing the cold zone in adiabatic adsorbers since Collins. Earlier suggestions included raising the feed temperature using external heating or through partial bypass of the feed compressor aftercooler.
Collins proposed the use of heat conducting rods or plates extending the length of the bed for the same purpose. Others have extended this concept by replacing the rods or plates with hollow tubes filled with liquid to provide heat transfer by convection between the warmer product end and the colder feed end of the adsorber. For example, the cold zone temperature is increased from xe2x88x9270xc2x0 C. to near 0xc2x0 C. in Fraysse et al. (U.S. Pat. No. 5,520,721) by supplying a heat flux to a passage between the pretreatment and main adsorbents. The primary intent in all of these methods is to elevate the minimum temperature near the feed inlet of the adsorber using direct and/or indirect heat exchange. The entire bed temperature is elevated along with the cold zone temperature when the feed is heated, however, and the overall size of the thermal gradient in the bed remains relatively unaffected.
Another approach attempts to match an adsorbent with a temperature that is most efficient for the desired separation. Typical of such teachings, an adsorbent bed is divided into layers that are maintained at different temperatures using embedded heat exchangers to affect distinct separations.
Armond (EP 0512781 A1) claims to inhibit the effect of the cold zone by selecting two unspecified adsorbents with high removal efficiency for N2, at xe2x88x9235xc2x0 C. to xe2x88x9245xc2x0 C. and at ambient temperature, respectively. The low temperature material is located near the feed inlet (but downstream of the pretreatment adsorbent) and is followed by the second material.
A main adsorbent, containing at least two layers, has been disclosed by Watson et al. (U.S. Pat. No. 5,529,610) for O2 production. Watson teaches that no commercially available adsorbent functions optimally over the large temperature gradient (as much as 100xc2x0 F.) that exists in the main adsorbent region of the bed. NaX zeolite, comprising from 20% to 70% of the total adsorbent volume, is chosen for the lowest temperature region of the bed due to its low capacity and high selectivity at such temperatures. The second layer is preferably CaX zeolite, although other high capacity, high nitrogen selectivity adsorbents are also proposed for this region.
Co-pending and commonly assigned application Ser. No. 08/546,325 to Leavitt et al. (now U.S. Pat. No. 5,674,311) discloses layered beds in which the adsorbent are selected according to optimum adsorption figures-of-merit (AFM) at particular temperatures in the bed. The figure-of-merit index is computed from equilibrium properties of the adsorbent. As with the teachings cited above, Leavitt teaches that one should address large thermal gradients (e.g. about 70xc2x0 F.) in an adsorber
Reiss teaches in U.S. Pat. No. 5,114,440 a VSA process for O2 enrichment of air using two or three layers of CaA zeolite of varying N2 capacity for the main adsorbent. The CaA adsorbents are arranged such that the material of lowest N2 capacity is placed near the feed inlet while that of highest N2 capacity is located near the product end of the adsorber. Power consumption was shown to be lower for the layered CaA adsorber as compared to adsorbers containing CaA of uniform N2 capacity and an adsorber containing NaX near the feed inlet followed by CaA near the product end.
JP Appl. No. 4-293513 teaches that improved stability of operation (less variability in bed size factor (BSF), power, and final desorption pressure) is achieved under varying ambient temperatures (xe2x88x9210xc2x0 C. to 40xc2x0 C.) in VPSA O2 production using a layered main adsorbent bed consisting of equal volumes of CaA and CaX zeolites when compared to adsorbers containing either of the individual adsorbents alone. The CaA zeolite is located near the feed end and is followed by the CaX adsorbent.
Multiple adsorbent layers have also been proposed in order to reduce the overall cost of product O2 Such an approach is disclosed in U.S. Pat. No. 5,203,887 (Toussaint), wherein a layer of less costly NaX replaces LiX adsorbent in a section of the main adsorbent nearest the product end. An alternative to this two-layer arrangement for the main adsorbent is the addition of a third layer (NaX) between the LiX and the pretreatment layer near the feed inlet of the adsorber.
Thus, the prior art has focused upon mitigating the apparent undesirable effects of the subambient cold zone through heat transfer means and/or by selection of an appropriate adsorbent for the low temperature region of the bed. Layering of adsorbents has been proposed as a means of improving separation efficiency in the presence of large bed temperature gradients (50-100xc2x0 F.). While the most commonly suggested adsorbents for the cold zone are NaX and CaA zeolites, a variety of adsorbents have been recommended for the regions of the bed beyond the cold zone.
It is therefore an object of the invention to provide a PSA process and apparatus that achieve improved efficiency, reduced cost and extended production ranges for PSA air separation processes using advanced adsorbents.
It is a further object of the invention to provide a PSA process and system that having no cold zone and consequently small (e.g. less than about 50xc2x0 F.) temperature gradients.
It is a further object of the invention to provide a PSA apparatus requiring no additional equipment for heat addition or removal from the adsorber.
The invention comprises a PSA process and apparatus wherein the fixed adsorbent bed comprises an equilibrium zone and a mass transfer zone. Further, the equilibrium and mass transfer zones each comprise at least one adsorbent material, selective for the adsorption of a more selectively adsorbable component, that is selected on the basis of the performance of that adsorbent material under the process conditions applicable to said zone.
In a preferred embodiment, at least one adsorbent material selected for the equilibrium zone is selected on the basis of said adsorbent material""s adiabatic separation factor for a gas mixture of two or more components.
In another preferred embodiment, at least one adsorbent material selected for either the equilibrium zone or the mass transfer zone is selected on the basis of said adsorbent material""s adiabatic separation factor for a gas mixture of two or more components.
In another preferred embodiment, at least one absorbent material selected for either the equilibrium zone or the mass transfer zone is selected in view of the different gas compositions in said zones during at least one of adsorption or desorption.
In still another preferred embodiment, at least one adsorbent material selected for the mass transfer zone has a comparatively high adiabatic separation factor for the more adsorbable material and a comparatively low adiabatic delta loading for the less adsorbable component under the process conditions applicable to said zone.
In another preferred embodiment, the gas mixture is air.
It should be noted that the terms xe2x80x9cworking capacityxe2x80x9d, xe2x80x9cdynamic capacityxe2x80x9d and xe2x80x9cdelta loadingxe2x80x9d as used herein are interchangeable. Also for the purposes of this invention, the property referred to by these terms is determined under adiabatic operation.